Joining Marine Fighter Attack Squadron 212 (VMFA-212) in the spring of 1972, his ensuing operational assignments included tours with VMFA-235, VMFA-232, and VMFA-122 where he flew the F-4 Phantom II. In the fall of 1978 Amos left the active Marine Corps to become a commercial pilot for Braniff International Airways. He was employed with Braniff for 25 months, returning to the Marine Corps in January 1981.[4]

After a three-year posting as a Flight Instructor in Advanced Jet Training, attendance at the Armed Forces Staff College in Norfolk, Virginia as a Major, and a 13-month overseas staff assignment to the III Marine Amphibious Force, Okinawa, Japan, he was transferred to Marine Aircraft Group 24, Kaneohe Bay, Hawaii. Promoted to Lieutenant Colonel, he assumed command of Marine Air Base Squadron 24, later re-designated as Marine Wing Support Squadron 173. Joining VMFA-212, in 1987, Lieutenant Colonel Amos deployed to the western pacific as the squadron's Executive Officer for what would be called "Operation Last Dance," the last overseas deployment of the Marine F-4 Phantom before it was phased out and retired.

Promoted to Brigadier General in 1998, Amos was assigned to NATO as the Deputy Commander, Naval Striking Forces, Southern Europe, Naples, Italy. During this tour, he commanded NATO's Kosovo Verification and Coordination Center in Skopje, Macedonia, and later served as the Chief of Staff, U.S. Joint Task Force Noble Anvil during the air campaign over Yugoslavia and Kosovo. Transferred to the Pentagon in the summer of 2000, Amos' other one-star assignments included posting as the Assistant Deputy Commandant for Aviation in 2000, and the Assistant Deputy Commandant for Plans, Policies and Operations in 2001.

In August 2002, he was promoted to Major General and assumed command of the 15,000 Marines of the 3rd Marine Aircraft Wing based out of Miramar, California leading them in combat twice between 2003 and 2004 during Operations IRAQI FREEDOM I and II. Receiving his third star in 2004, Lieutenant General Amos assumed command of the 45,000+ Marines and Sailors of the II Marine Expeditionary Force at Camp Lejeune, NC. In 2006 he was reassigned as the Commanding General, Marine Corps’ Combat Development Command, and as the Deputy Commandant for Combat Development and Integration, at Quantico, VA, where he was responsible for the training of all Marines, their combat units, their continuing education, and for the identification of all Marine equipment requirements.

Receiving his fourth star in July 2008, he assumed duties as the 31st Assistant Commandant of the Marine Corps, based at the Pentagon in Arlington, Virginia.[5] In June 2010, Amos was recommended for nomination by U.S. Secretary of DefenseRobert Gates to become the 35th Commandant of the Marine Corps. He was nominated by PresidentBarack Obama on July 20,[6] who interviewed him for the job on June 17.[7][8] and confirmed by the Senate Armed Services Committee on September 21,[9] and confirmed shortly thereafter. On October 22, 2010, at a ceremony at Marine Barracks, Washington, D.C. he assumed the position as Commandant.[10] This marked the first time a Marine aviator has held the position of Commandant, and the first time since 1983 that a sitting Assistant Commandant had moved up to become Commandant.[citation needed]

Amos exchanges the Battle Colors of the Marine Corps with incoming Commandant General Joseph Dunford on October 17, 2014

During his tenure as Commandant, he completed the Corps’ combat mission in Afghanistan. Facing sequestration and the significant effects of the Budget Control Act, he fought to re-balance the Corps during the period of marked fiscal austerity, all while ensuring the combat readiness of the Marine Corps for its continued role as our nation's Expeditionary Crisis Response Force. Responding to multiple global challenges that had been critically acclaimed as "the new normal," he established Special Purpose - Marine Air Ground Task Force-Crisis Response commands in both Africa and the Middle East, while adding 1,000 Marine Security Guards to America's embassies around the world.[11][12] Additionally, during his final two years as Commandant, he shepherded the Corps’ considerable efforts to address head-on its many challenges in recruiting and maintaining a diverse and talented body of Marines.[13] Lastly, he significantly raised the requirements and capacity within the Marine Corps University for all Marines to attend resident Professional Military Education. .[14]

In November 2010, shortly after becoming Commandant, Amos, along with the Chief of Naval Operations, and the Chief of Staff of the Air Force, were notified of an over $4 billion shortfall in the JSF Program. Reacting to this and the steadily rising costs and delays in the program, Secretary of Defense Robert Gates called a meeting with Secretary of the Navy Ray Mabus and Amos, ostensibly to cancel the Marine variant, the F-35B. Amos was successful in convincing Gates of the Corps' need for the aircraft, choosing rather to place the F-35B on a two-year probation to allow the program to mature and right itself. Amos' detailed and hands-on actions over the next 18 months ensured positive progress on the Marine variant in most areas of concern, resulting in Secretary Leon Panetta removing the F-35B from probation six months early. As a result, the Marine Corps was the first service to be able to stand up and operationally certify a Joint Strike Fighter squadron within the DoD.[17][18][19]

After a decade plus of declining amphibious ship numbers in the U.S. Navy inventory, and increasing operational requirements for Marines around the world, Amos partnered with the Chief of Naval Operations, the Navy Secretariat, and industry to reconfirm amphibious ship requirements, acceptable readiness levels, and total hull numbers. Working closely with the United States Congress, in the Senate and the House over two years of budgetary efforts, Congress authorized and appropriated monies to buy a, not-programmed, 12th San Antonio Class LPD amphibious war ship. Additionally, a memorandum of understanding was signed by the Chief of Naval Operations, the Secretary of the Navy, and Amos agreeing to recapitalizing the Navy's more than 40-year-old LSD amphibious ships with the new, and proven, San Antonio Class hull design. This single agreement alone will result in millions of dollars saved over the lifetime of the LSD replacement effort, and will accelerate the completion and delivery of the ships years ahead of schedule.[20]

In 2012, Amos partnered with key legislators In the House and the Senate to bring national recognition to the service and sacrifices of the Corps' WWII African American "Montford Point Marines" with the awarding of the Congressional Gold Medal.[not in citation given] African-American men were recruited into the Marine Corps by Presidential order beginning 1942. Establishing a segregated Boot Camp on a swampy point of land in Jacksonville, NC known as Montford Point, the Marine Corps trained some 20,000 African American men between 1942 and 1949. Commandant of the Marine Corps, General Alexander Vandegrift, closed down the segregated training in 1949, stating "The experiment with the negro Marines is over. They are Marines …Period!" The Montford Point Marines served in the Pacific during WWII, and again in 1951 during the Korean War. Their service and faithfulness to our nation had never been successfully recognized at the national level until Amos initiated service-level efforts to have them awarded the Congressional Gold Medal.[21] Final authorization was signed and the medal struck on November 23, 2011, recognizing the Montford Point Marines.[22][23]

Sensing that 10 years of sustained combat may have begun fraying the edges of the Corps' moral fabric, Amos initiated an effort to morally and ethically "reground the Corps." The effort, named the "Reawakening", targeted the Corps' central leadership cadre, the Non-Commissioned Officer (NCO). Amos and Sergeant Major Michael Barrett, the Sergeant Major of the Marine Corps, spent the better part of 2013 traveling to most every Marine base to personally challenge the Corps' NCO leadership with getting back to the basics. The sole focus of the Reawakening was simply to remind Marines of their higher calling. Amos reinforced "who they were" as Marines, "what they did for the nation", and "who they were not".[24]

As Commandant, Amos opposed the repeal of the "don't ask, don't tell" policy regarding homosexuals openly serving in the U.S. military.[25] After President Obama signed the legislation setting the conditions for repeal, Amos led the Department of Defense in carrying out the will of the nation's civilian leadership.[26] In late November 2011, Amos stated that his opposition to gays openly serving in the military has proven unfounded and said that Marines have embraced the change, describing the repeal as a "non-event."[27]

In August 2014, the Marine Corps Times wrote, "Amos was cleared in November of allegations he showed preferential treatment to the son of his predecessor as commandant, a field grade officer who held a senior leadership position with the scout snipers’ parent command. Then last month the inspector general determined Weirick was not the subject of reprisal when he was removed from his job for confronting one of Amos’ legal advisers.[32]

Amos also faced allegations of interference in the sexual assault case against Staff. Sgt. Steve Howell.[35] The case was overturned on the basis of Amos's 'appearance of unlawful command influence, rather than actual command influence.'[36]